A great deal of work has been performed in recent years to develop cost-effective methods for generating ethanol from biomass. The use of biomass to generate ethanol for fuel presents several advantages over the use of more traditional feedstock sources. The potential raw materials are abundant and diverse, the use of these feedstocks does not divert from the food supply, and they potentially exhibit a smaller carbon footprint.
Although biomass provides an attractive substrate for ethanol production, it also presents several challenges. First, biomass contains both cellulose, which can be broken down into the hexose sugar glucose, and hemicellulose, which can be broken down into both hexose sugars and pentose sugars such as xylose and arabinose. Many of the microorganisms traditionally used in ethanol fermentation are incapable of fermenting both hexose and pentose sugars to ethanol. Second, unlike more traditional sources of ethanol feedstock (e.g., corn, cane sugar), biomass includes structural components from plant sources. Because the source material is structural and more difficult to break down, biomass requires more processing to generate the sugar monomers that function as a fermentation substrate. Third, hydrolysate resulting from pre-treatment of biomass presents a harsh environment for fermenting microorganisms.
Several bacterial species are capable of fermenting pentose sugars to ethanol, but these species generally produce a mixture of products rather than a single product. Often one or more of these products are harmful to the bacteria. Further, bacteria can exhibit drastically reduced fermentation rates in the harsh environment of plant matter hydrolysate.
Yeast are generally considered to be more attractive candidates for industrial-scale ethanol fermentation than bacteria. However, very few yeast are capable of fermenting pentose sugars to ethanol. Various genetic modifications have been introduced into different yeast species in an attempt to overcome this problem. However, none of these previously developed modified strains have proven entirely satisfactory for large-scale ethanol production from biomass. Therefore, there is a need in the art for new genetically modified yeast strains capable of fermenting biomass to ethanol.